Radio communication systems rely on modulating carrier frequencies in a finite portion of the electromagnetic spectrum to wirelessly transmit and receive signals. Modulation can be performed on the amplitude, frequency, and/or phase of the carrier signal to separate the carrier from unwanted noise. The information portion of the signals typically conveys information such as voice, video, and computer data between transmitting/receiving devices such as voice terminals (e.g., wireless phone) and data terminals (e.g., portable computer).
In order to transmit the signals over a large distance, a relay such as a satellite may be used. Passive communication satellites may be used to return transmitted signals to earth on the same carrier frequency they were transmitted on. Active communication satellites can be used to receive the transmitted signals on one carrier frequency and to retransmit the signals on another carrier frequency. Geosynchronous satellites are especially well suited for such a task due to their stationary position relative to the earth's surface.
A rapid increase in the number of voice/data terminals that are in service, as well as new data intensive applications, has created a demand for a corresponding increase in the capacity and speed of many communication systems. Advances in underlying technology and changes in communication regulations allow communication systems to be implemented with increased capacity, increased coverage, higher speeds, lower costs, and smaller equipment. However, a practical problem with changing over from older to newer technologies is that large investments in equipment retard the transition to state of the art equipment. For example, large investments in military equipment operating in the Ultra High Frequency (UHF) band require on-going heritage interoperability with new digital systems thereby requiring new investments in digital beam forming satellite equipment operating in L and S bands.